1. Field of Invention
The present invention relates to fuel cell, and more particularly to an integral multi-stack system of fuel cell which requires low operating air pressure and has low weight, manufacturing cost, and power consumption. Also, the integral multi-stack system can be simply and quickly disconnected for replacement of one or more fuel cell stacks.
2. Description of Related Arts
A fuel cell is an electrochemical energy conversion device that produces electricity and heat during the process of converting hydrogen and oxygen into water. The technology is promising because it offers a means of making power more efficiently with less pollution. Fuel cells are usually classified by the type of electrolyte being used. The proton exchange membrane PEM fuel cell is one of the most promising technologies. This is the type of fuel cell that will be powering cars, buses and houses in future.
FIG. 1 is a schematic view showing the working of a single PEM fuel cell 8. The single fuel cell 8 comprises a membrane electrode assembly MEA 81, a first field flow plate 82, and a second field flow plate 83. The MEA 81 comprises electrodes anode and cathode; not shown in the FIG. 1 with a thin layer of catalyst, bonded to either side of a PEM. Hydrogen 70 flows through passages 821 defined in the first field flow plate 82 to the anode where a platinum catalyst promotes its separation into protons and electrons. Hydrogen 70 may be supplied to a fuel cell engine directly or can be obtained from natural gas or methanol using a fuel processor.
Air 60, or pure oxygen, is supplied to the cathode of the MEA 81 through a plurality of passages defined in the second flow field plate 83 on a side opposite the viewer. Oxygen in the air 60 attracts the hydrogen protons through the PEM. The air steam also removes water created as a by-product of the electrochemical process.
This reaction in a single fuel cell 8 produces only limited volts. To get this voltage up to a desired level, many separate fuel cells must be combined to form a fuel-cell stack. Referring to FIG. 2, a type of conventional PEM fuel-cell stack 6 is illustrated. The fuel-cell stack 6 has a stack unit 69 including a plurality of single fuel cells 8 piled up together, a first end plate 67 and a second end plate 67′ placed at a top side and a bottom side of the stack unit 69 respectively, a plurality of long insulate screws 671 pulling the first and second end plates 67, 67′ closer with respect to each other so as to tightly sandwich the stack unit 69 for support, a pair of current collectors 68, 68′ electrically connected to all anodes and cathodes, respectively, of all single fuel cell 8, and a plurality of manifolds and ports for carrying and directing reactants and coolant. The fuel-cell stack 6 as shown in FIG. 2 has six such manifolds and corresponding ports including a fuel inlet 63, an oxidant inlet 62, a coolant inlet 61, an exhausted fuel outlet 66, an exhausted oxidant outlet 65, and an exhausted coolant outlet 64, all of which are arranged on the second end plate 67′. Each such port 61, 62, 63, 64, 65, or 66 can be arranged on any end plate 67 or 67′. For some low-power fuel-cell stacks which do not need special cooling, the manifolds and corresponding ports for coolant supply and discharging can be saved. The end plates 67, 67′ are generally required to be non-conductive and non-porous.
In actual use, in order to achieve a voltage, a current, or a power which is much greater than one stack can does, more than one stacks need to be placed close together in terms of physical space, facilitating an electrical parallel or series connection. Such fuel cell stacks may be fueled by a common main hydrogen passage and a main air passage, and cooled down by common main water passages. This technology of establishing a multi-cell system is usually called integration of fuel cell stacks. FIG. 3 shows a typical conventional way of integration in which a multi-cell system 9 has a first end frame 91 and a second end frame 92 which sandwich more than one stack units 69 therebetween and are pulled towards each other by a plurality of bolts 93. A hydrogen supply passage, an air supply passage, and a water supply passage are defined in the first end frame 91, without communication with each other respectively for supplying the hydrogen, air, and cooling water needed by the stack units 69. A hydrogen discharge passage, an air discharge passage, and a water discharge passage for respectively discharging the hydrogen, air, and cooling water are defined in the second end frame 92. For each stack unit 69 in the multi-cell system, a hydrogen supply sub-passage, an air supply sub-passage, and a water supply sub-passage are defined in the first end frame 91 for air communication and connecting the hydrogen supply passage, the air supply passage, the water supply passage respectively to the hydrogen inlet, the air inlet, and the cooling water inlet. A hydrogen discharge sub-passage, an air discharge sub-passage, and a water discharge sub-passage are provided in the second end frame 92 in an air communicating manner to connect the hydrogen discharge passage, the air discharge passage, and the water discharge passage respectively to the hydrogen outlet, the air outlet, and the cooling water outlet of each stack unit 69. All passages and sub-passages do not communicate with each other.
However, such kind of integration has two main drawbacks. First, for a stack unit 69 made up by a certain number of single fuel cells 8, any flow must pass a whole length of the stack unit 69 from the first end frame 91 to the second end frame 92, making a pressure heads thereof drop substantially. In fact, pumps must be employed to provide the pressure required to overcome the loss of pressure head. It inevitably increases the weight, the manufacturing cost, and the power consumption of the whole equipment. Second, when one of the stack units 69 is out of work and needs to be disassembled and repaired, all bolts 93 must be loosened and one end plate 91 or 92 removed before the problem stack unit 69 can be taken out. It brings much inconvenience for using of such a multi-stack system.